Site-specific mutagenesis of the N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6- phenylimidazo[4,5-b]pyridine DNA adduct in mammalian cells

Site-specifically modified oligodeoxynucleotides were used to explore the mutagenic properties of a cooked food mutagen-derived DNA adduct, N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6-phenylimidazo[4,5-6]pyridine (dG-C8-PhIP). A dG-C8-PhIP-modified oligodeoxynucleotide was prepared post-synthetically by reacting an oligodeoxynucleotide containing a single dG (^5′-TCC TCC TCG CCT CTC T) with N-acetoxy-PhIP. The unmodified and dG-C8-PhIP-modified oligomers were inserted into single-strand (ss) phagemid vectors. These ss vectors were transfected into simian kidney (COS-7) cells. The progeny plasmid obtained was used to transform Escherichia coli DH10B. The transformants were analyzed by oligodeoxynucleotide hybridization and sequencing to determine the mutation frequency and spectrum. Preferential incorporation of dCMP, the correct base, was observed opposite dG-C8-PhIP. Targeted mutants showing G→T transversions were detected, along with a small number of G→A transitions and G→C transversions. Significant amounts of non-targeted mutations representing C→T transitions also were detected 5′ to the dG-C8-PhIP lesion. Thus, dG-C8-PhIP, a major DNA adduct induced by PhIP, is mutagenic in mammalian cells and may be involved in the initiation of human cancer.